The present invention relates to a permanent magnet rotating electric machine that has a rotor built with a plurality of permanent magnets embedded in a rotor core.
A conventional permanent magnet rotating electric machine is disclosed, for example, in JP-A-9-327140. In such a permanent magnet rotating electric machine, the rotor core is fixed on the shaft and the rotor is built by inserting a plurality of permanent magnets, each with a rectangular cross section, from the shaft direction into the storage section formed on the rotor core so that the rotor can rotate with a predetermined gap to the inner periphery of the stator core within the stator. The permanent magnets are magnetized such that the north pole and the south pole alternate.
A motor with rectangular magnets embedded in the rotor, such as the one described above, is efficient during high-speed rotation because field weakening is effective during high-speed rotation. For this reason, the motor is used, for example, as the permanent magnet motor on an electric car where high-speed rotation is required. To avoid vibrations and noises generated during driving, the magnets within the rotor are split in the longitudinal direction to produce semi-slot skews for attaining low-torque pulsation.
The motor torque of a magnet-embedded rotating electric machine such as the one described above is expressed by expression (1) given below.
T=xcfx86Iq+(Lqxe2x88x92Ld)Iqxc3x97Idxe2x80x83xe2x80x83(1)
where, T is the motor torque, xcfx86 is the magnetic flux of the permanent magnet, Lq is the q-axis inductance, Ld is the d-axis inductance, Iq is the q-axis winding current, and Id is the d-axis winding current.
In expression (1), the first term is the torque of the major magnetic flux of the permanent magnet, and the second term is a reluctance torque generated by the auxiliary magnetic pole of an iron core between two magnets. The magnetic torque has a period for each pole pair (electrical angle of 360 degrees), while the reluctance torque has a period for each pole (electrical angle of 180 degrees).
In this case, if the rotor is skewed as in the prior art example, the maximum torque is decreased because of a difference in the current phase for generating the maximum torque. If the magnet torque is the main torque, the torque decrease is small, for example, about 5% because the magnetic torque has a period of 360 degrees; however, if the reluctance torque is the main torque, the torque decrease is large, for example, about 10% because the reluctance torque has a period of 180 degrees. Thus, for a motor that uses the reluctance torque as the main toque, there has been a need for a shape that reduces torque pulsation with no skewing.
It is an object of the present invention to provide a permanent magnet rotating electric machine that uses no skewing under a predetermined current and voltage condition to prevent torque from decreasing and, at the same time, decreases pulsation torque to make the machine less vibrating and less noisy.
(1) To achieve the above object, a permanent magnet rotating electric machine according to the present invention comprises a stator on which multi-phase stator windings are provided and a rotor which is built by embedding a plurality of permanent magnets internally into a rotor core and which is rotatably arranged with a predetermined gap to the stator, a core shape of the rotor being uniform in a depth (longitudinal) direction, wherein a plurality of permanent magnets are arranged for each pole of the rotor and wherein the plurality of permanent magnets are symmetrical with respect to a rotation direction but irregular with respect to the depth direction.
This configuration allows a permanent magnet rotating electric machine, with no skewing under a predetermined current and voltage condition, to prevent torque from decreasing and to decrease pulsation torque, thus making the machine less vibrating and less noisy.
(2) Preferably, in (1) described above, there are, for each pole, three or more permanent-magnet-inserting holes through which the permanent magnets are inserted into the rotor.
(3) Preferably, in (1) described above, the ratio of the length of one permanent magnet arrangement to the length of another permanent magnet arrangement in the depth direction of the rotor is 1:1.
(4) Preferably, in (1) described above, xcex8 satisfies a relation xcex8=(n+0.5)xc3x97xcfx84s+xcfx86 (n is an integer) wherein, for each pole of the rotor, xcex8 is an angle between between-pole permanent-magnet-inserting holes with its vertex at a center of a shaft, xcfx86 is an angle of a pole-center permanent-magnet-inserting hole with its vertex at the center of the shaft, and xcfx84s is a slot pitch.
(5) Preferably, in (1) or (4) described above, a magnetic flux generated from between-pole magnets equals a magnetic flux generated from a pole-center permanent magnet for each pole of the rotor.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.